Using 3D Bezier Curve but having different shading
Is your curve made from a single continuous segment, or does it have multiple separate segments?

If it has multiple separate segments some of them may have a different direction, yielding inverted normals. You can either:


Invert the curve direction of the affected segments in Edit Mode, by pressing Switch Direction button in the tool shelf, or
Tick the Calc Order in the Screw modifier to make them consistent automatically




If you already converted it to a mesh or applied all modifiers just enter edit mode, select all faces and press Ctrl+N to recalculate normals.